Water is made of two hydrogen atoms and one oxygen atom bonded together. julia is describing how water undergoes a physical change when it boils. she says that the particles shift to be farther apart and that the oxygen and hydrogen separate. she also says that the total mass of the steam is the same as the initial water, and that heat energy was required to cause the change. she finishes her description by saying that the steam can be turned back into water if it is cooled.

which is the mistake in her description?

the total mass remains the same in physical changes.
the arrangement of particles may change in physical changes.
atoms remain bonded to each other in physical changes.
energy changes may occur in physical changes.

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.